An R—Fe—B based anisotropic sintered magnet, including an Nd2Fe14B type compound phase as a main phase, is known as a permanent magnet with the highest performance, and has been used in various types of motors such as a voice coil motor (VCM) for a hard disk drive and a motor for a hybrid car and in numerous types of consumer electronic appliances. When used in motors and various other devices, the R—Fe—B based anisotropic sintered magnet should exhibit thermal resistance and coercivity that are high enough to withstand an operating environment at an elevated temperature.
As a means for increasing the coercivity of an R—Fe—B based anisotropic sintered magnet, a molten alloy, including a heavy rare-earth element RH as an additional element, may be used. According to this method, the light rare-earth element RL, which is included as a major rare-earth element R in an R2Fe14B phase, is replaced with a heavy rare-earth element RH, and therefore, the magnetocrystalline anisotropy (which is a decisive quality parameter that determines the coercivity) of the R2Fe14B phase improves. However, although the magnetic moment of the light rare-earth element RL in the R2Fe14B phase has the same direction as that of Fe, the magnetic moments of the heavy rare-earth element RH and Fe have mutually opposite directions. That is why the remanence Br would decrease in proportion to the percentage of the light rare-earth element RL replaced with the heavy rare-earth element RH.
The metal structure of an R—Fe—B based anisotropic sintered magnet consists essentially of an R2Fe14B phase, which is a main phase, and a so-called “R-rich phase” that has a relatively high R concentration and a low melting point, but also includes an R oxide phase and a B-rich phase (R11.1Fe4B4 phase). Those additional phases, other than the main phases, are collectively called “grain boundary phases”. In this case, it is the main phase that contributes to increasing the coercivity by substituting the heavy rare-earth element RH. On the other hand, the heavy rare-earth element RH, included in those grain boundary phases, will not directly contribute to increasing the coercivity of the magnet.
Meanwhile, as the heavy rare-earth element RH is one of rare natural resources, its use is preferably cut down as much as possible. For these reasons, the method in which a portion of the light rare-earth element RL is replaced with the heavy rare-earth element RH in the entire magnet (i.e., over not only the whole main phase but also the grain boundary phases) is not preferred.
To get the coercivity increased effectively with the addition of a relatively small amount of the heavy rare-earth element RH, it was proposed that an alloy or compound powder, including a lot of the heavy rare-earth element RH, be added to a powder of main phase material alloy including a lot of the light rare-earth element RL and then the mixture be compacted and sintered. According to this method, the heavy rare-earth element RH is distributed a lot in the outer periphery of the main phase grain, and therefore, the magnetocrystalline anisotropy of the R2Fe14B phase can be improved efficiently. The R—Fe—B based anisotropic sintered magnet has a nucleation-type coercivity generating mechanism. That is why if a lot of the heavy rare-earth element RH is distributed only in the outer periphery of the main phase (i.e., near the grain boundary thereof), the magnetocrystalline anisotropy of all crystal grains is improved, the nucleation of reverse magnetic domains can be interfered with, and the coercivity increases as a result. At the core of the main phase crystal grains, no light rare-earth element RL is replaced with the heavy rare-earth element RH. Consequently, the decrease in remanence Br can be minimized there, too. Such a technique is disclosed in Patent Document No. 1, for example.
If this method is actually adopted, however, the heavy rare-earth element RH has an increased diffusion rate during the sintering process (which is carried out at a temperature of 1,000° C. to 1,200° C. on an industrial scale) and could diffuse to reach the core of the main phase crystal grains, too. For that reason, it is not easy to obtain the expected crystal structure in which the heavy rare-earth element RH is included in increased concentrations in only the outer periphery of the main phase.
As another method for increasing the coercivity of an R—Fe—B based anisotropic sintered magnet, a metal, an alloy or a compound including a heavy rare-earth element RH is deposited on the surface of the sintered magnet and then thermally treated and diffused. Then, the coercivity could be recovered or increased without decreasing the remanence so much.
Patent Document No. 2 teaches forming a thin-film layer, including R′ that is at least one element selected from the group consisting of Nd, Pr, Dy, and Tb on the surface of a sintered magnet body to be machined and then subjecting it to a heat treatment within either a vacuum or an inert atmosphere, thereby turning a deformed layer on the machined surface into a repaired layer through a diffusion reaction between the thin-film layer and the deformed layer and recovering the coercivity.
Patent Document No. 3 discloses that a metallic element R (which is at least one rare-earth element selected from the group consisting of Y, Nd, Dy, Pr, and Tb) is diffused to a depth that is at least equal to the radius of crystal grains exposed on the uppermost surface of a small-sized magnet while the thin film is being deposited, thereby repairing the damage done on the machined surface and increasing (BH)max.
Patent Document No. 4 discloses that by depositing a CVD film, consisting mostly of a rare-earth element, on the surface of a magnet with a thickness of 2 mm or less and then subjecting it to a heat treatment, the rare-earth element would diffuse inside the magnet, the machined and damaged layer in the vicinity of the surface could be repaired, and eventually the magnet performance could be recovered.
Patent Document No. 5 discloses a method of sorbing a rare-earth element to recover the coercivity of a very small R—Fe—B based sintered magnet or its powder. According to the method of Patent Document No. 5, a sorption metal, which is a rare-earth metal such as Yb, Eu or Sm with a relatively low boiling point and with a relatively high vapor pressure, and a very small R—Fe—B based sintered magnet or its powder are mixed together, and then the mixture is subjected to a heat treatment to heat it uniformly in a vacuum while stirring it up. As a result of this heat treatment, the rare-earth metal is not only deposited on the surface of the sintered magnet but also diffused inward. Patent Document No. 5 also discloses an embodiment in which a rare-earth metal with a high boiling point such as Dy is sorbed (see Paragraph #0014 of Patent Document No. 5). In such an embodiment that uses Dy, for example, Dy is selectively heated to a high temperature by induction heating (with no temperature conditions specified). However, Dy has a boiling point of 2,560° C. According to Patent Document No. 5, Yb with a boiling point of 1,193° C. should be heated to a temperature of 800° C. to 850° C. but could not be heated sufficiently by normal resistance heating process. Considering this disclosure of Patent Document No. 5, it is presumed that the Dy be heated to a very high temperature. For example, to achieve a Dy vapor pressure that is almost as high as the vapor pressure at the Yb heating condition (of 800° C. to 850° C.) that is defined as a preferred temperature to advance the sorption favorably, Dy should be heated to approximately 1,800° C. to approximately 2,100° C. It is also disclosed that as for Yb, its sorption is realized at approximately 550° C. and Yb has a vapor pressure of about 10 Pa in that case. This value corresponds to the saturation vapor pressure of Dy at 1,200° C. That is to say, if Dy should be sorbed by the technique disclosed in Patent Document No. 5, then Dy should be heated to at least 1,200° C., and preferably to 1,800° C. or more. It should be noted that the saturation vapor pressures of respective elements are known physical property values. Patent Document No. 5 also states that according to any heating condition, the temperature of the very small R—Fe—B based sintered magnet and its powder is preferably kept within the range of 700° C. to 850° C.
And Patent Document No. 6 discloses a technique for improving the magnetization property, while reducing the amount of Dy used, by mixing together a material alloy powder with a relatively high Dy concentration and a material alloy powder with a relatively low Dy concentration and subjecting the mixture to a sintering process.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2002-299110        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 62-74048        Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2004-304038        Patent Document No. 4: Japanese Patent Application Laid-Open Publication No. 2005-285859        Patent Document No. 5: Japanese Patent Application Laid-Open Publication No. 2004-296973        Patent Document No. 6: Japanese Patent Application Laid-Open Publication No. 2002-356701        